Project Summary/Abstract The long-term goal of this proposed research plan is to understand how aging affects the representation of sound frequency in the human auditory system. Age-related declines in processing of frequency information may contribute to decreased speech perception, even among older adults whose hearing sensitivity can be described as falling within clinical normal limits. These declines in the auditory system are thought to be, in part, related to how central auditory nervous system represents or encodes acoustic characteristics of sounds, such as frequency. The proposed experiment will use behavioral and physiological measures to evaluate age- related changes in the processing of frequency information. In addition, the relationship between perceptual and physiological measures will be examined. The purpose of Aim 1 is to characterize the effects of age on the ability to perceive differences between sounds at each ear. Two behavioral tasks will be completed. First, the upper frequency limit of interaural phase difference (IPD) detection will be established. Second, interaural phase difference limens will be obtained at 500, 750, 1000, 1125, and 1250 Hz. The purpose of Aim 2 is to characterize the effects of age on the neural representation of frequency in monaural (one ear) and binaural (both ears) conditions, as measured with the frequency-following response (FFR). The FFR is an auditory evoked potential that is sensitive to the frequency content of stimuli. FFRs will be obtained at 500, 750, 1000, 1125, and 1250 Hz from the same participants that IPD limens were obtained from to address specific aim one. The purpose of Aim 3 is to determine the effects of age on the relationship between the perception and neural representation of frequency and interaural phase differences, as well as speech-in-noise understanding. From a basic science perspective, the proposed research will help define the biological process of aging on the perception and neural representation of frequency in the presence of noise, where older adults have the greatest difficulty understanding speech. From a clinical perspective, information gained from the proposed research plan may help to improve approaches to aural rehabilitation of adults with hearing loss by providing a better understanding of contributing factors. It is hypothesized that older adults will have poorer perception and neural representation of frequency, as well as poorer speech-in-noise understanding.